Weekly Report: Influenza Summary Update

On June 11, the World Health Organization raised the pandemic alert level from Phase 5 to Phase 6 indicating that an influenza pandemic is underway.

Synopsis:

During week 29 (July 19-25, 2009), influenza activity decreased in the United States; however, there were still higher levels of influenza-like illness than is normal for this time of year.

Six hundred forty-three (16.1%) specimens tested by U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories and reported to CDC/Influenza Division were positive for influenza.

A total of 5,514 hospitalizations and 353 deaths associated with novel influenza A (H1N1) viruses have been reported to CDC.

Over 98% of all subtyped influenza A viruses being reported to CDC were novel influenza A (H1N1) viruses.

The proportion of deaths attributed to pneumonia and influenza (P&I) was below the epidemic threshold.

Two influenza-associated pediatric deaths were reported; both deaths were associated with novel influenza A (H1N1) virus infection.

The proportion of outpatient visits for influenza-like illness (ILI) was below national and region-specific baseline levels.

Four states and Puerto Rico reported geographically widespread influenza activity, eight states reported regional influenza activity, 19 states and the District of Columbia reported local influenza activity, and 19 states reported sporadic influenza activity.

WHO and NREVSS collaborating laboratories located in all 50 states and Washington D.C. report to CDC the number of respiratory specimens tested for influenza.

During the 2008-09 influenza season, influenza A (H1), A (H3), and B viruses have co-circulated in the United States. On April 15 and 17, 2009, CDC confirmed the first two cases of novel influenza A (H1N1) virus infection in the United States, and this virus has been the predominant circulating influenza strain since testing and reporting of novel influenza A (H1N1) viruses by U.S. WHO collaborating laboratories began during week 17 (week ending May 2, 2009).

Because of the extensive spread of novel influenza A (H1N1) within the United States, it has become extremely resource intensive to count individual cases. In addition, since only a small proportion of persons with respiratory illness are tested for novel influenza A (H1N1) infection, confirmed and probable case counts represent a significant underestimate of the true number of novel influenza A (H1N1) cases in the U.S. As a result, CDC will no longer report individual case counts; only hospitalizations and deaths associated with confirmed novel influenza A (H1N1) will be reported each week on the CDC H1N1 influenza website (http://www.cdc.gov/h1n1flu/update.htm)

As of July 31, 2009, 5,514 hospitalizations and 353 deaths (7 deaths in individuals 0-4 years, 56 deaths in individuals 5-24 years, 142 deaths in adults 25-49 years, 96 deaths in adults 50-64 years, 29 deaths in adults age 65 and older, and 23 deaths with unknown age) associated with novel influenza A (H1N1) virus have been identified by CDC and state and local public health departments. The results of tests performed during the current week are summarized in the table below.

Week 29

No. of specimens tested

3,988

No. of positive specimens (%)

643 (16.1%)

Positive specimens by type/subtype

Influenza A

641 (99.7%)

A (novel H1N1)

381 (59.4%)

237 (37.0%)

A (unable to subtype)

16 (2.5%)

A (H3)

4 (0.6%)

A (H1)

3 (0.5%)

Influenza B

2 (0.3%)

During week 29, seasonal influenza A (H1), A (H3), and B viruses co-circulated at low levels with novel influenza A (H1N1) viruses. Over 98% of all subtyped influenza A viruses being reported to CDC this week were novel influenza A (H1N1) viruses.

The increase in the percentage of specimens testing positive for influenza by WHO and NREVSS collaborating laboratories may be due in part to changes in testing practices by health care providers, triaging of specimens by public health laboratories, an increase in the number of specimens collected from outbreaks, and other factors.

All 1,155 seasonal influenza A (H1) viruses are related to the influenza A (H1N1) component of the 2008-09 influenza vaccine (A/Brisbane/59/2007). One hundred ninety-three (96%) of 201 influenza A (H3N2) viruses tested are related to the A (H3N2) vaccine component (A/Brisbane/10/2007) and eight viruses (4%) tested showed reduced titers with antisera produced against A/Brisbane/10/2007.

All 242 novel influenza A (H1N1) viruses are related to the A/California/07/2009 (H1N1) reference virus selected by WHO as a potential candidate for novel influenza A (H1N1) vaccine.

Influenza B viruses currently circulating can be divided into two distinct lineages represented by the B/Yamagata/16/88 and B/Victoria/02/87 viruses. Seventy-two (12%) of 625 influenza B viruses tested belong to the B/Yamagata lineage and are related to the vaccine strain (B/Florida/04/2006). The remaining 553 (88%) viruses belong to the B/Victoria lineage and are not related to the vaccine strain.

Data on antigenic characterization should be interpreted with caution given that antigenic characterization data is based on hemagglutination inhibition (HI) testing using a panel of reference ferret antisera and results may not correlate with clinical protection against circulating viruses provided by influenza vaccination.

Annual influenza vaccination is expected to provide the best protection against those virus strains that are related to the vaccine strains, but limited to no protection may be expected when the vaccine and circulating virus strains are so different as to be from different lineages, as is seen with the two lineages of influenza B viruses. Antigenic characterization of novel influenza A (H1N1) viruses indicates that these viruses are antigenically and genetically unrelated to seasonal influenza A (H1N1) viruses, suggesting that little to no protection would be expected from vaccination with seasonal influenza vaccine.

Antiviral Resistance:

Since October 1, 2008, 1,128 seasonal influenza A (H1N1), 222 influenza A (H3N2), and 635 influenza B viruses have been tested for resistance to the neuraminidase inhibitors (oseltamivir and zanamivir). Also, 1,133 seasonal influenza A (H1N1) and 224 influenza A (H3N2) viruses have been tested for resistance to the adamantanes (amantadine and rimantadine). Two hundred seventy-four novel influenza A (H1N1) viruses have been tested for resistance to the neuraminidase inhibitors (oseltamivir and zanamivir). Three hundred twenty-one novel influenza A (H1N1) viruses have been tested for resistance to the adamantanes (amantadine and rimantadine). The results of antiviral resistance testing performed on these viruses are summarized in the table below.

Isolates tested (n)

Resistant Viruses, Number (%)

Isolates tested (n)

Resistant Viruses, Number (%)

Oseltamivir

Zanamivir

Adamantanes

Seasonal Influenza A (H1N1)

1,128

1,123 (99.6%)

0 (0)

1,133

6 (0.5%)

Influenza A (H3N2)

222

0 (0)

0 (0)

224

224 (100%)

Influenza B

635

0 (0)

0 (0)

N/A*

N/A*

Novel Influenza A (H1N1)

274

0 (0)

0 (0)

321

321 (100%)

*The adamantanes (amantadine and rimantadine) are not effective against influenza B viruses.

The novel influenza A (H1N1) virus is susceptible to both neuraminidase inhibitor antiviral medications zanamivir and oseltamivir. It is resistant to the adamantane antiviral medications, amantadine and rimantadine. Antiviral treatment with either oseltamivir or zanamivir is recommended for all patients with confirmed, probable or suspected cases of novel influenza A (H1N1) virus infection who are hospitalized or who are at higher risk for seasonal influenza complications.

Rare sporadic cases of oseltamivir resistant novel influenza A (H1N1) viruses have been detected worldwide, none of which were detected in the United States. Additional information on antiviral recommendations for treatment and chemoprophylaxis of novel influenza A (H1N1) infection is available at http://www.cdc.gov/h1n1flu/recommendations.htm

Three seasonal influenza A (H1N1) viruses collected between February 8 and May 11, 2009, were found to be resistant to both oseltamivir and the adamantanes (amantadine and rimantadine). All influenza A (H1N1) viruses tested retain their sensitivity to zanamivir. The three dually-resistant viruses represent less than 0.3% of all seasonal influenza A (H1N1) viruses tested during the 2008-09 influenza season, and as a result, no changes to the influenza antiviral treatment or prophylaxis recommendations will be made at this time. CDC will continue to monitor trends in antiviral resistance over the summer and throughout the upcoming 2009-10 influenza season.

Two influenza-associated pediatric deaths were reported to CDC during week 29 (Florida and Utah). Both of these deaths were associated with novel influenza A (H1N1) virus infection. The deaths reported this week occurred during weeks 23 and 26 (the weeks ending June 13, 2009 and July 4, 2009). Since September 28, 2008, CDC has received 97 reports of influenza-associated pediatric deaths that occurred during the current influenza season, 29 of which were due to novel influenza A (H1N1) virus infections.

Of the 41 children who had specimens collected for bacterial culture from normally sterile sites, 16 (39.0%) were positive; Staphylococcus aureus was identified in 10 (62.5%) of the 16 children. Four of the S. aureus isolates were sensitive to methicillin and six were methicillin resistant. Fourteen (87.5%) of the 16 children with bacterial coinfections were five years of age or older and 10 (62.5%) of the 16 children were 12 years of age or older. Ten of the 29 children with confirmed novel influenza A (H1N1) infection had a specimen collected from a normally sterile site; two of the 10 children had a positive bacterial culture (methicillin resistant S. aureus and Streptococcus constellatus). An increase in the number of influenza-associated pediatric deaths with bacterial coinfections was first recognized during the 2006-07 influenza season. In January 2008, interim testing and reporting recommendations were released regarding influenza and bacterial coinfections in children and are available at
(http://www2a.cdc.gov/HAN/ArchiveSys/ViewMsgV.asp?AlertNum=00268).

Laboratory-confirmed influenza-associated hospitalizations are monitored in two population-based surveillance networks: the New Vaccine Surveillance Network (NVSN) and the Emerging Infections Program (EIP).

During October 12, 2008 to July 11, 2009, the preliminary laboratory-confirmed influenza-associated hospitalization rate for children 0-4 years old in the NVSN was 4.42 per 10,000. Because of case identification methods utilized in this study, there is a delay from the date of hospitalization to the date of report.

Data collection for influenza-associated hospitalizations through the NVSN has been completed for the 2008-09 influenza season. There will be no updates to this system after week 28.

During April 15, 2009 to July 25, 2009, the following preliminary laboratory-confirmed overall influenza associated hospitalization rates were reported by the EIP (rates include type A, type B, and novel H1N1):

Rates for children aged 0-23 months, 2-4 years, and 5-17 years were 2.0, 0.8, and 0.6 per 10,000, respectively. Rates for adults aged 18-49 years, 50-64 years, and >= 65 years were 0.4, 0.4, and 0.4 per 10,000, respectively.

*This value represents an age group-specific average influenza rate from October 1 to April 30 from the 2005-06, 2006-07, and 2007-08 influenza seasons. **Note: The scales for the 0-23 month and the >= 65 years age groups differ from other age groups.

Nationwide during week 29, 1.1% of patient visits reported through the U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet) were due to influenza-like illness (ILI). This percentage is below the national baseline of 2.4%.

On a regional level, the percentage of outpatient visits for ILI ranged from 0.3% to 2.0%. All 10 regions reported percentages of visits for ILI below their respective region-specific baselines. ILI decreased during week 29 in seven of 10 regions compared to week 28.

Geographic Spread of Influenza as Assessed by State and Territorial Epidemiologists:

The influenza activity reported by state and territorial epidemiologists indicates geographic spread of both seasonal influenza and novel influenza A (H1N1) viruses and does not measure the severity of influenza activity.

During week 29, the following influenza activity was reported:

Widespread influenza activity was reported by Puerto Rico and four states (California, Hawaii, Maine, and New Jersey).

Regional influenza activity was reported by 8 states (Arizona, Arkansas, Florida, Georgia, New York, North Carolina, Pennsylvania, and South Carolina).

Local influenza activity was reported by the District of Columbia and 19 states (Alaska, Connecticut, Iowa, Maryland, Massachusetts Michigan, Minnesota, Montana, Nevada, New Hampshire, Oregon, Rhode Island, Tennessee, Texas, Virginia, Washington, West Virginia, and Wyoming).